This invention relates to a safety relief system for a pilot operated relief valve in a vent or main flow line, and more particularly to such a system having a main relief valve and associated pilot valve particularly adapted for use at temperatures over around 350.degree. F. and as high as 1200.degree. F.
Heretofore, safety relief systems such as shown in U.S. Pat. No. 4,425,938, dated Jan. 17, 1984 have been provided in which a main safety relief valve has been mounted over a pressure vessel, such as a tank, with a main valve member having a dome in fluid communication with an associated pilot valve which senses upstream fluid pressures in the tank or flow line for controlling the movement of the main valve member. The main valve member moves to an open position upon the reaching of a predetermined high fluid pressure to relieve or reduce the fluid pressure within the pressure vessel to a predetermined low level.
Further, so-called no-flow or non-flowing pilot valves have been utilized heretofore in combination with main relief valves for controlling the opening and closing of the main valve member. A non-flowing pilot valve does not have a continuous fluid flow during its normal operating or on position when the main valve member is in a static condition. The elimination of a continuous fluid flow through the pilot valve removes many of the problems encountered in pilot valves having a continuous fluid flow such as, for example, the clogging of orifices or ports with entrained foreign matter in the flowing fluid, or icing resulting from wet gas applications. A non-flowing pilot valve normally includes an intermediate fluid chamber between the inlet fluid chamber and the outlet fluid chamber so that simultaneous fluid flow is not provided through the inlet and outlet fluid chambers. However, non-flowing pilot valves in such systems heretofore have normally utilized elastomeric seals which are adapted for use at relatively low temperatures below around 350.degree. F. as elastomeric seals tend to deteriorate at temperatures over around 350.degree. F.-400.degree. F.
Normally, a safety relief valve mounted over a tank, for example, has a body chamber with a main valve member mounted therein for reciprocal movement with its inner end surface exposed to fluid pressure from the tank through an inlet passage. Upon the reaching of a predetermined high pressure in the tank, the reciprocal valve member such as a piston moves outwardly or vertically upwardly to an open position to relieve or reduce the fluid pressure in the tank to a predetermined low level by fluid flow through the main relief valve. Thus, in closed position, the body chamber of the main relief valve is exposed to outlet fluid pressure which may be atmospheric.
A dome chamber is normally provided over the main valve member which is in fluid communication with the intermediate chamber of the associated pilot valve, and the fluid pressure in the dome chamber is generally the same as the inlet fluid pressure. Thus, the main valve member is responsive to fluid inlet pressure which is the pressure of the pressure vessel. In order that the dome pressure remains generally the same as the inlet fluid pressure, it is necessary that a fluid tight seal be provided for the reciprocal valve member so that leakage does not occur past the reciprocal valve member from the dome chamber to the body chamber to reduce the fluid pressure in the dome pressure. Normally, an elastomeric seal is provided between the main valve member and the adjacent body member to provide such a fluid tight seal, but elastomeric seals can not be subjected to high temperatures above around 400.degree. F. without substantial deterioration and associated leakage, particularly at temperatures above 600.degree. F.